Curvature and acoustic instabilities in rotating fluid disks.
Item
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Title
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Curvature and acoustic instabilities in rotating fluid disks.
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Identifier
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AAI9912608
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identifier
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9912608
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Creator
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Montenegro, Luis Enrique.
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Contributor
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Adviser: Chi Yuan
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Date
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1998
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Language
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English
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Publisher
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City University of New York.
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Subject
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Physics, Astronomy and Astrophysics
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Abstract
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The stability of a rotating fluid disk to the formation of spiral arms is studied in an improved version of the tightwinding approximation in the linear regime. Commonly ignored terms such as the radial variation of basic disk properties and multiple spiral arms change significantly the stability of the disk especially at small galactrocentric radii, and are included in this study. Two dimensionless parameters are found to be important: the ratio of disk mass to total mass (disk + halo), and the ratio of epicyclic radius to disk radius. Disks that may be stable to axisymmetric perturbations are generally unstable to spirals. For such disks, perturbations grow at a rate proportional to a linear combination of these two parameters. Three types of new instabilities are identified: gravitational-curvature instabilities, acoustic-curvature instabilities, and acoustic instabilities. The acoustic instabilities arise from the asymmetric nature of the spirals and from the differential rotation present in the disk.;Unstable growth rates are determined for these instabilities in several representative cases with varying types of differential rotation and self-gravity. The most important applications appear to be as a source of spiral structure in non-self-gravitating disks with short orbital times.;Observations with the Hubble Space Telescope reveal an irregular network of dust spiral arms in the nuclear region of the interacting disk galaxy NGC 2207. Properties of the gas imply that none of the usual spiral-generating mechanisms are present. Instead, the spirals may result from acoustic instabilities that amplify at small galactic radii.;Multiple armed stellar spirals are evident in computer enhanced images of disk spiral galaxies. The images reveal clear superpositions of simple two- and three-arm structures. The radial extents of both spiral structures are consistent with the modal theory that most two-arm spirals are self-sustaining wave modes, the three-arm spirals are driven by asymmetries in the two-arm wave modes, and are a likely source of energy loss from the two-arm mode. The three arm spirals may take several revolutions to build up once an asymmetric two arm spiral appears, thus the morphology of the galaxy will slowly change with time.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
